Rendering isn't stressing GPU
Not all scenes will use 100% of the GPU, or 100% of the CPU or 100% of available Memory. Blender will use the computer resources depending on the scene's complexity. Simple scenes will not use the same resources as complicated ones.

3D rendering can be a very complex and resource hungry task, but a wide variety of factors determine render time and computer usage: The number of polygons and subdivisions, types of shaders, modifiers, number of textures, texture size, use of volumetrics, particles or physics simulations, the number of samples, bounces, the size of the rendered tiles, amount and speed of RAM, etc, etc and blah, blah, blah... Not all operations are performed by the GPU so even when rendering on GPU you'll have to wait for the CPU to complete certain processes.

Again, depending on the scene, cutting render time by one half might just be unrealistic. To render in almost half the time, the most viable solution is using two GPUs instead of one...

Finding the optimal settings for your system is a trial and error process. What works on one scene might not work for others. Test different tile sizes (start around 256x256 and see how that goes), play with the number of samples and options in path tracing, simplify, optimize, decimate, bake, use all possible tricks to keep things to your liking, with tolerable noise and within realistic rendering times.

In short: There is no one-size-fits-all solution when it comes to cutting render time.